KOCHAB (Beta Ursae Minoris). Kochab, an obscure Arabic name that
might simply mean "star," is just barely the second brightest and
thus appropriately the Beta star in Ursa
Minor, the Smaller Bear, and represents the top front bowl star
of the Little Dipper. Second magnitude (2.08, ranking 59th
brightest in the
sky) and only 16 degrees from the North
Celestial Pole, middle northerners can see it every night as it
plies its small circular daily path. Together with the other bowl
star (Pherkad, Gamma Ursae Minoris), it
makes a small asterism called the "Guardians of the Pole," the two
seeming in myth to "protect" the so-important pole star. Though we are familiar with the major two
motions of the Earth, daily rotation and annual revolution, the
third motion, precession, is more
obscure. The Moon and Sun act on the Earth's rotational bulge, and
cause the axis to wobble over a 26,000 year period. The result is
that the axis continually moves in a circle of 23 1/2 degrees
radius against the background stars. Polaris is therefore only a
temporary pole star that will get better into the next century and
will then begin to shift away. About the year 1100 BC, the pole
made a reasonably close pass to Kochab, and there are old
references to Kochab being called "Polaris." Precession also
causes the Vernal Equinox (where we find
the Sun on the first day of spring) to move backwards through the
constellations of the Zodiac; the equinox is now in Pisces rather than in Aries where it was when the constellations were being
named. Unlike the Sun, Kochab has run out of internal hydrogen
fuel, and is an evolving orange class K (K4) giant star that is either
brightening with a dead helium core or is dimming after already
having fired its internal helium as it prepare to be an ordinary
helium burner like so many class K giants. At a distance of 131
(second Hipparcos reduction) light years (give or take just 1), it
shines with a luminosity of 450 times that of the Sun. It appears about the same brightness as
much more luminous Polaris because it is much closer and because,
at a temperature of 4130 degrees Kelvin, it radiates a fair amount
of its light in the infrared
where we cannot see it. It has a reputation as a marginal "barium
star," the element only a small bit enhanced relative to what is
found in the Sun, maybe accreted from an undetected close companion
that has already gone through its evolution and is now a white
dwarf. Then again, maybe not. It does seem to be a bit low in
iron, about 60 percent that found in the Sun (relative to
hydrogen). Luminosity and temperature yield a radius of 42 times
that of the Sun, very close to the value of 44 solar found from
interferometer measures of angular diameter. Theory suggests a
mass around 3 Suns, though analysis of subtle oscillations give a
lower, and probably more accurate, mass of 1.3 Suns. At a
separation of 3.5 minutes of arc lies an 11th magnitude "companion" whose rapid motion
shows it merely to lie in the line of sight.